This invention is directed to an improved process for preparing polyarylates which are melt stable and low in color.
Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)propane also identified as Bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyacrylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diester derivative, which is then reacted with an aromatic dicarboxylic acid(s), to form the polyarylate.
U.S. Pat. No. 4,075,173 issued Feb. 21, 1978, describes the preparation of copolyesters by reacting an aromatic dicarboxylic acid, a diacetate of Bisphenol-A, and an acetate of p-hydroxybenzoic acid. Various processes for producing polyarylates by the reaction of Bisphenol-A and terephthalic and isophthalic acids are reviewed in this patent. The following process for producing polyarylates, identified as route (1), is described in column 2, of the patent: ##STR1##
This process is the diacetate process as described herein, or the "Acetate Process" as defined in the patent.
Column 2 of the patent states:
"The route (1) is not desirable because the undesirable coloration and deterioration of polymer are particularly remarkable as disclosed in the above-mentioned literature."
Further, column 3 of the patent states:
"On the other hand, the route (1), Acetate process, is economically advantageous because the materials used are cheap and the operation is simple For example, diacetate of bisphenol-A, a monomer for Acetate process, is synthesized by merely reacting acetic anhydride and bisphenol-A. Consequently, it may be said that, if the fatal drawbacks of Acetate process, color and deterioration, are solved, Acetate process will become the most superior process."
Thus, the skilled workers in the field of polyarylate chemistry realize that the existing processes for producing polyarylates have one or more deficiencies, and that a need exists to develop a viable diacetate process for producing polyarylates.
The process of said U.S. Pat. No. 4,075,173 requires the use of p-hydroxybenzoic acid and produces a particular class of polyarylate copolymers.
In the following U.S. Patent Applications, novel methods for producing polyarylates by the diacetate process are described. Specifically these U.S. Patent applications describe the following:
U.S. Patent Application Ser. No. 069,818, filed Aug. 27, 1979 in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates in the Presence of a Diphenyl Ether" describes a process for preparing polyarylates having a reduced viscosity of from about 0.5 to greater than 1.0 dl/gm, which process comprises reacting at least one diester derivative of a dihydric phenol with at least one aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent, based on the polyarylate produced, of a diphenyl ether compound, at a temperature of from about 260.degree. to about 350.degree. C.
U.S. Patent Application Ser. No. 070,039, filed Aug. 27, 1979 in the name of L. M. Maresca, et al. and titled "Process For Preparing Polyarylates in the Presence of a Diphenyl Ether Compound and A Catalyst" describes a process for preparing polyarylates which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent of a diphenyl ether compound at a temperature of from about 260.degree. to about 350.degree. C. and in the presence of a magnesium catalyst.
U.S. Application Ser. No. 069,819, filed Aug. 27, 1979 in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates" describes a process for preparing polyarylates of improved color which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen atom, at a temperature of from about 260.degree. to about 350.degree. C. Optionally, the process may be carried out in the presence of a magnesium, manganese, or zinc catalyst.
U.S. Patent Application Ser. No. 126,994, filed Mar. 3, 1980 in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates" describes a process for preparing polyarylates which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of from about 10 to about 60 weight percent of at least one halogenated and/or etherated substituted aromatic or heteroaromatic compound, at a temperature of from about 260.degree. to about 350.degree. C.
However, in said U.S. Patent Applications Ser. Nos. 069,818; 070,039; 069,819 and 126,994, supra, the polyarylates produced by the diacetate process described therein still tend to contain colored species to an unacceptable extent and tend to be melt unstable if the intermediate dihydric phenol diester is not carefully purified prior to polymerization. Thus, the polyarylate must be prepared from a highly purified intermediate dihydric phenol diester, or it is difficult to fabricate. Also without purification of the diester, the polyarylate may not be acceptable in applications where polyarylates which are low in color are required.
It has now been unexpectedly found that removal of residual acid anhydride from the crude dihydric phenol diester reaction product prior to reaction with an aromatic dicarboxylic acid(s) to form the polyarylate polymer consistently produces a polyarylate which is melt stable and low in color. The residual acid anhydride is removed so that its concentration is less than about 1500 parts per million. Excess acid anhydride is necessary in the reaction with the dihydric phenol since it provides higher conversions in shorter time periods.